In such a circuit the magnetic component may be storing energy supplied from the circuit or, when supply to the component is removed, itself supplying previously stored energy to the circuit. During the supply phase, when the component is driven, the voltage output of the sense winding is determined by the supplied drive and the sense winding may be used to measure that drive. During the so-called flyback phase, when stored energy is returned to the circuit, the magnetising current in the component will fall as the energy transfer occurs. The rate at which this occurs depends upon the circuit loading and so, during this time, the output of sense winding may be used to measure circuit behaviour.
In many applications, these sensed voltages will be used to control or regulate a system of which the magnetic component forms part. An advantage of the sensing arrangement is that both voltages may be sensed, albeit at different times, by means of a single connection. Where the controller comprises an integrated circuit device, this means a single connecting pin to the device, which is an important cost saving consideration. However, there are some complications to be addressed in the implementation of such a system in practice.
By the nature of flyback, the voltages to be measured will be of opposite polarity. Moreover, it is likely that the magnitude of the voltage to be measured during the flyback phase will be significantly less than that of the voltage to be measured during the drive phase. Where the magnetic component is a transformer driven on a primary side and producing an output on a secondary side, this situation may be exacerbated since the flyback phase voltage may be divided by the turns ratio of the transformer. Thus there can be significant disparity in the dynamic range of the two measurements. This dynamic range issue detracts from the inherent advantage of single pin sensing and separate sensing arrangements may be required for comparable accuracy in the two measurements. This problem becomes particularly severe and potentially unworkable where a control integrated circuit device operates at a low single ended supply voltage with the sensed voltage biased to give single polarity measurements. Here the poor resolution to which the output voltage (5V, for example, perhaps stepped down by 10:1) can be sensed once the full potential range of the input (90-260 V, for example) is accommodated may be unacceptably low.
One field in which use of a magnetic sense winding is attractive is in power converters. Power converter designs are often based on magnetic coupling between a primary drive circuit fed from a mains derived unregulated dc supply and a secondary output circuit which supplies the load because of the electrical isolation between the primary side and the secondary side. The attraction of the sense winding is that it allows the output on the secondary side to be sensed (for example for the purpose of regulation) without compromising isolation. Indeed, sense windings are already commonly used in power converters, especially for output over-voltage detection. Such a winding is almost always needed for bias generation within the converter in any event.
Unfortunately, the difficulties in using a single sense winding during both drive and flyback that follow from the dynamic range disparity are ruling its use out in many applications. The poor resolution to which the output voltage can be sensed may be unacceptably low for successful regulation to be achieved. For this reason, designs which employ separate secondary side sensing schemes that provide a more accurate measurement of output voltage are being used. Such schemes do not have the isolation advantage of the sense winding and signals are typically fed back to the primary side controller through opto-couplers which arrangements add significantly to the cost of the converter.